It’s a question that seems to come up over and over again among left-leaning Americans: How, given all of the things — they would argue racist and sexist — that Donald Trump has said, is this race as close as it seems? How has an experienced politician like Clinton not buried her opponent by now?

Some blame the media for taking it too easy on Trump and giving him all kinds of free publicity — or for not calling him out more directly for telling lies. But that theory ignores that fact that Trump has been, and remains, the most unpopular presidential candidate in modern American history. As many as two-thirds of Americans don’t like him.

And yet, he’s still close. Why? Why is Clinton not much further ahead in a race that her supporters — and many in the media — thought was un-lose-able?

A few reasons:

1) Rank partisanship

If you're a major-party presidential nominee in this day and age in American politics, you’ve frankly got to work pretty hard to get less than 40 or even 45 percent of the vote. Because we’re just that partisan.

A presidential race has been decided by double digits only once since 1972. And since that Richard Nixon-reelecting landslide in ‘72, neither party has taken less than 40 percent of the vote in a two-candidate race. In fact, neither side has taken less than 45 percent in a two-candidate race since Ronald Reagan. (This excludes two of the past seven races, of course — both Bill Clinton victories featuring Ross Perot as an independent candidate. But it likely would have held up in Perot’s absence.)

In fact, if you think back to 2008, Barack Obama's 7-percentage-point popular vote victory was treated as a landslide.

Part of the reason is that the two parties themselves have become more ideologically polarized. The day of the conservative Southern Democrats and the liberal Northern Republicans have long passed. The Democratic Party is now clearly the party of the left and the Republicans of the right.

The result is that voters in states and districts around the country are voting for the same party for president, Senate, U.S. House and many other offices. While more than 100 congressional districts voted for a different party’s candidate for president and House in every election between 1956 and 1996, just 25 did so in 2012.

Via Vital Statistics on Congress

The point is that, whether through the parties becoming more polarized or people becoming more polarized (or both), voters are much more predictable when it comes to definitely pulling the lever for the Republican Party or the Democratic Party.

Trump certainly has his faults — and his record-high unfavorable rating proves it — but he hasn’t alienated that 40-plus percent. And he probably never will.

2) Enthusiasm

Beyond predictable partisanship, there’s the matter of turnout and who can actually get their supporters to the polls. And when it comes to doing that, it helps to have enthusiasm. Right now, Trump may have more of it.

A Washington Post-ABC News poll earlier this month showed that while 46 percent of Trump backers were “very enthusiastic" about voting, just 33 percent of Clinton backers said the same. And 93 percent of Trump backers were certain to vote in November, as compared with 80 percent of Clinton backers.

For whatever reason, Clinton voters have mostly just seemed less excited about this election. And that’s perhaps why the growing number of polls surveying “likely voters” rather than “registered voters” show numbers for Trump. The average of all registered-voter polls on Huffington Post Pollster shows Clinton up by 3. The average of all likely-voter polls, though, shows her up by 1.

Would more enthusiasm suddenly mean Clinton would be ahead by 10 points? No. But it’s arguably making this a closer race for the time being. And it’s also a reflection of another factor that is preventing a Clinton blowout win, which is ...

3) Hillary Clinton

The big reason this election isn’t a blowout right now may be Hillary Clinton herself. Trump’s image numbers are bad enough that a candidate with even middling numbers of his or her own would probably be leading him by a substantial margin. But Clinton’s numbers are also bad, and it makes the race close.

In fact, the most recent Washington Post-ABC News poll showed about as many registered voters had an unfavorable opinion of her (59 percent) as disliked Trump (60 percent). Her favorable rating (38 percent) was also basically indistinguishable from his (37 percent).

Clinton supporters, of course, will gladly blame these numbers on the media’s coverage of their candidate — particularly her email server and the Clinton Foundation, which they believe to be overblown. But her numbers have been in steady decline for years — with a brief respite during her time as a highly popular secretary of state.

Clinton is running against a highly flawed opponent who has done plenty to hurt himself in this race. She also happens to be running to continue the policies of a suddenly quite popular incumbent Democratic president in Barack Obama. So perhaps she should be further ahead. (Obama himself has offered some theories as to why she isn’t, including partisanship and sexism. “This should not be a close election, but it will be, and the reason it will be is not because of Hillary’s flaws,” he told supporters at a recent fundraiser.)

But the idea that this race would ever have been a complete blowout for her was always far-fetched. It’s just not how politics in this country work these days, and it ignores the fact that Clinton has long been a very polarizing politician in her own right.